As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity; and the sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a continuous tube in known manner.
The tube of packaging material, which in effect forms an extension of the aseptic chamber, is fed in a vertical direction, is filled with the sterilized or sterile-processed food product, and is fed through a sealing device to form the individual packages. More specifically, in the sealing device, the tube is sealed at a number of equally spaced cross sections to form pillow packs connected to one another by transverse sealing strips, i.e. extending perpendicularly to the travelling direction of the tube; and the pillow packs are separated from one another by cutting the relative transverse sealing strips, and are conveyed to a folding station where they are folded mechanically to form respective finished parallelepiped-shaped packages.
Packaging machines are known, as described for example in European Patent EP-B-0887265, which comprise two chain conveyors defining respective endless paths and fitted with respective numbers of sealing jaws. The two paths have respective branches substantially facing and parallel to each other, and between which the tube of packaging material is fed so that the jaws on one conveyor cooperate with corresponding jaws on the other conveyor along said branches of the respective paths, to grip the tube at a number of successive cross sections, and to seal and cut the packs.
Packaging machines are also known comprising only two pairs of jaws, which act alternately on the tube of packaging material to grip and seal, e.g. heat seal, it along a number of equally spaced cross sections.
Once the sealing operation is completed, a cutter, carried, for example, by one of the jaws in each pair, is activated, and interacts with the tube of packaging material to cut it along a centre line of the cross section just sealed, and so detach a pillow pack from the bottom end of the tube of packaging material. The bottom end being sealed transversely, the relative jaws, on reaching the bottom dead-centre position, can be opened to avoid interfering with the top portion of the tube. At the same time, the other pair of jaws, operated in exactly the same way, moves down from the top dead-centre position, and repeats the above grip/form, seal and cut process.
In both types of packaging machines, the tube portion gripped between each pair of jaws is normally sealed by heating means carried on one of the jaws and which locally melt the layers of heat-seal plastic material gripped between the jaws.
Ultrasound sealing devices are now widely used to locally melt the packaging material faster and so increase output.
Ultrasound sealing devices substantially comprise a mechanical-vibration generator, or sonotrode, and an anvil—as described, for example, in EP-B-615907—which are fitted to respective jaws, and have respective surfaces which cooperate with each other to heat the packaging material by ultrasound vibration.
More specifically, a sonotrode is a sealing tool which is vibrated by one or more disks of piezoelectric material; the disks are supplied with alternating voltage, and generate mechanical vibration of an energy depending on the actual supply voltage or electric current supply.
A need is felt within the industry to prevent the formation, at the sealing stage, of polyethylene blisters or lumps, which impair sealing quality and, in some cases, may even pierce the barrier material layer, thus impairing the aseptic nature of long-storage products.